Vortex shedding produced by passing a fluid through a cylindrical object is a typical physical problem in the field of fluid dynamics, and vortex shedding has caught much attention and related efforts have been conducted in the past years. Since a change of fluid pressure is created when the vortex is shed, therefore a change of fluid pressure can be used for measuring the vortex shedding frequency which can be used for calculating the fluid flow rate. The vortex shedding has a well-known property that a linear relationship exists between the vortex shedding frequency of a fluid and the flow rate of the fluid within an appropriate range of Reynolds number, and such property is used by the industry as a vortex flowmeter for detecting the vortex shedding frequency.
Referring to FIG. 1 for an exploded view of a conventional vortex flowmeter 1, the vortex flowmeter 1 comprises a vortex shedder 2, a sensor 3 and a circular pipe 4, wherein the vortex shedder 2 is comprised of a flat plate 21 perpendicular to a bluff body 22, and the vortex shedder 2 has a T-shape cross-section for improving the quality of a vortex shedding signal (as disclosed in R.O.C. Pat. No. 071182 and U.S. Pat. No. 5,351,559); the bluff body 22 has a pressure hole 221 disposed separately at appropriate positions on both lateral sides of the bluff body 22, and the pressure hole 221 is connected to a through hole 222; and the sensor 3 has a probe 31 inserted into the through hole 222 of the vortex shedder 2, such that the probe 31 of the sensor 3 can detect the pressure of a fluid turbulence caused by the vortex shedding process through the pressure hole 221, and the sensor 3 can detect the vortex shedding frequency by the pressure change of the fluid. The sensor 3 sends the signal with the measured vortex shedding frequency to an external electronic device, and the electronic device filters and processes the signal transmitted from the sensor 3 to obtain the flow rate of the fluid and the volume flow rate; the circular pipe 4 has a penetrating hole 41 disposed at an appropriate position for fixing the vortex shedder 2, and the circular pipe 4 is connected to a pipeline having a fluid flow, such that the fluid passing through the circular pipe 4 produces shedding vortices by the vortex shedder 2.
The vortex shedding signal detected by the sensor 3 is usually affected by noises when the conventional vortex flowmeter 1 measures the vortex shedding frequency and the signal outputted from the sensor 3 is transmitted to the external electronic device, the noises are filtered by a filter circuit before the signal is used for calculating the vortex shedding frequency. However, the filtering effect of the electronic device is poor if the vortex shedding signal is measured by the sensor 3 at a low noise ratio. As a result, errors will be produced when the electronic device measures the vortex shedding frequency, and the system will become unstable for the automatic flow control.